The invention relates to a storage medium for thermally-assisted magnetic recording including a recording layer having substantially parallel tracks for recording information. The invention also relates to a method for recording information on a storage medium and an apparatus for carrying out the method.
In the near future, both longitudinal magnetic and (magneto) optical recording will be hampered in their growth to higher densities and data rates. In conventional magnetic recording superparamagnetism will finally limit the stable and low-medium-noise recording of information, while in magneto-optical recording the bit size and read data rate are limited by the optical resolution and the limited Kerr rotation and allowable laser power during reading, respectively. It is generally believed that the limits of conventional magnetic disk recording of about 50-100 Gb/in2 will be reached in a few years from now, because the present areal density in the most advanced products is already near 10 Gb/in2 and the present annual growth rate is over 60%.
Limiting areal densities, Da, of 50-100 Gb/in2 (1 Gb/in2=1 Giga bit per square inch=1.6 bit/xcexcm2) for conventional longitudinal recording were estimated by Bertram et al.1, based on the calculated SNR of granular media, SNRmed, with 10-year data life time and recordable with high-saturation write heads. For future improvements in densities a drastic reduction of the grain size is necessary to maintain a sufficient SNRmed. Particle-size reduction, however, reduces the stability of the stored information strongly, unless the anisotropy field, Hk, of the particles is drastically increased. A medium with a high Hk has a higher coercivity field, Hc, and it is more difficult or even impossible to record on such media with conventional write heads or even the best write heads presently available with iron or cobalt-rich flux guides.
The areal density may be further increased using hybrid recording. Hybrid recording is a form of thermally-assisted magnetic recording. Both hybrid recording and magneto-optical recording use a magnetic field to change locally the magnetisation of the recording layer and a radiation beam to heat the recording layer. In general, the radiation beam has a wavelength in or near the visible part of the spectrum. In hybrid recording the bits are recorded along a track of a recording layer in the form of magnetisation transitions by reversing a magnetic field at the position of the recording layer, wherein the position of the transition along the track is determined by the reversals of the magnetic field. In contrast, the position of the magnetisation reversals when using magneto-optical recording is determined by the change in power of the radiation beam used to heat the recording layer. In other words, the position of the transition is fixed by a decrease of the magnetic field in hybrid recording and by a decrease in temperature in magneto-optical recording. The sharpness of the magnetisation transition in hybrid recording is determined by the magnetic field gradient in the recording layer during the recording process, whereas in magneto-optical recording it is determined by the temperature gradient in the recording layer during the recording process. In general the thermal profile in the recording layer caused by the radiation beam is larger than the distribution of the magnetic field in the recording layer in hybrid recording and smaller in magneto-optical recording.
The increase in density is limited by the stability of the recorded bits. In order to increase the stability of the recorded information on extremely small grains, writing in hybrid recording is carried out at an elevated temperature on a medium with a very high coercivity at room temperature, using write heads with high-saturation flux guides and an integrated light path added to it. Local heating of the medium by laser light through a light path during writing reduces the coercivity of the medium temporarily to a value that makes recording with high-saturation write heads possible. With hybrid recording, a smaller transition width and higher density or a better signal-to-noise ratio can be obtained than those achievable by conventional magnetic and magneto-optical recording.
Hybrid recording media are known from inter alia the article by H. Katayama, S. Sawamura, Y. Ogimoto, J. Nakajima. K. Kojima and K. Ohta, published in Proc. of MORIS""99, J. Magn. Soc. Jpn. 23, Suppl. S1, 233 (1999). A disadvantage of these media is that the storage life of the recorded data is shorter than what is determined from the storage temperature and the stability of the magnetic transitions on the recording layer.
It is an object of the invention to provide a hybrid recording medium having a storage life of the recorded data than the known recording media.
The object of the invention is achieved if the storage medium of the preamble is characterised in that the recording layer includes a series of recording regions, each region comprising a plurality of tracks having a pitch p for magnetically recording information, and extending a distance xc2xd p beyond a centre line of the outermost track of the region, and in that neighbouring regions are separated by magnetically non-recording areas having a width substantially equal to or larger than the pitch p. The invention is based on the insight, that the relatively short storage life of the known storage media for thermally-assisted magnetic recording is caused by the spatial extent of the thermal profile of the known recording heads. Since the profile extends over the tracks neighbouring the one being recorded, the neighbouring tracks are subject to short periods of increased temperature. The stability of the recorded data is limited by the total heating time during and after recording. Hence, recording of a track reduces the life of the data recorded in the neighbouring tracks. Repetitive recording of parts of the medium that are very close to each other will decrease the storage life of the data. When this recording is more or less randomly organised, the maximum writing time is more or less undetermined, and no specific maximum writing time can be guaranteed. Therefore, a special data write architecture is preferred to avoid heating much longer than the unavoidable heating time due to writing the tracks once. The storage medium according to the invention solves this problem by dividing the recording layer into regions separated from one another by non-recording areas. If the regions are written in a single session, the tracks in the region have been heated only during the unavoidable heating time. The neighbouring regions have not been heated at all, because of the non-recording areas. The regions can be written in a random fashion without endangering the data recorded in neighbouring regions.
The width of the non-recording areas should be at least equal to half the width of the thermal profile in the recording layer. For a small radiation distribution the distance between the centre lines of the closest tracks of neighbouring regions should be equal to or larger than twice the pitch of the tracks within a region. For larger thermal profiles, the distance should be at least three times the pitch.
A second aspect of the invention relates to an apparatus for thermally-assisted magnetic recording of information in the form of magnetisation transitions in tracks of a recording layer of a storage medium, the apparatus including a recording head for forming a radiation field and a magnetic field at the location of the recording layer, the radiation field forming a thermal profile larger than a distribution of the magnetic field in the recording layer, a control unit for controlling the radiation field and the magnetic field such that the position along the track of the magnetisation transitions are determined by reversals of the magnetic field. The size of the thermal profile for the purpose of comparison with the distribution of the magnetic field is the area where the temperature is higher than the average of room temperature and the maximum temperature in the thermal profile. Similarly, the size of the distribution of the magnetic field is the area where the magnetic field is larger than half the value of the maximum intensity of the magnetic field in the distribution.
A hybrid recording apparatus is known from inter alia the article by H. Katayama, S. Sawamura, Y. Ogimoto, J. Nakajima. K. Kojima and K. Ohta, published in Proc. of MORIS""99, J. Magn. Soc. Jpn. 23, Suppl. S1, 233 (1999).
In the second aspect, a hybrid recording apparatus is provided that can achieve a higher areal density on storage media than the known apparatus.
This is achieved if the apparatus is characterised in that the radiation field is pulsed synchronously with the magnetic field. In the known thermally assisted recording methods, the position of the magnetisation transitions in the recording layer is determined by the decrease of the pulsed magnetic field. Since the behaviour of the radiation field is regarded as unimportant for the position and sharpness of the magnetisation transition, the radiation field is kept at a constant power and the thermal profile in the recording layer is determined by the velocity with which the recording head moves over the recording medium. The solution is based on the insight, that in a hybrid recording process the position and the sharpness of the transitions can be improved considerably by increasing the cooling of the recording layer. This is achieved by changing from a continuous radiation field to a pulsed radiation field.
The pulse time of the radiation beam is preferably shorter than 5/f0, where f0 is the reversal attempt frequency of the recording layer. More preferably, the pulse time is shorter than 1/f0.
The pulse time is preferably smaller than 0.7 B/v in an apparatus for recording marks having a length B along the track, in which an actuator moves the storage medium at a velocity v with respect to the recording head.
In a preferred embodiment of the apparatus, the radiation field is elongate with a longest dimension in the track direction. The shape of the radiation field can advantageously be made using a recording head including a planar optical waveguide for forming the radiation field, the waveguide having an elongate exit window facing the location of the recording layer, the longest dimension of the exit window being oriented in the track direction.
In a preferred embodiment of the apparatus the recording head includes a magnetic read head.
The second aspect of the invention also relates to a method for thermally-assisted magnetic recording of information in the form of magnetisation transitions in tracks of a recording layer of a storage medium by imposing a radiation field and a magnetic field on the recording layer, the radiation field forming a thermal profile larger than a distribution of the magnetic field in the recording layer, and the position along the track of the magnetisation transitions being determined by reversals of the magnetic field, in which the radiation field is pulsed synchronously with the magnetic field.
A third aspect of the invention relates to an apparatus for thermally-assisted magnetic recording of information in the form of marks in tracks of a recording layer of a storage medium, the apparatus including a recording head for forming a radiation field and a magnetic field at the location of the recording layer, a magnetic read head, and an actuator for moving the recording medium relative to the recording head such that the recording head follows the tracks, the radiation field forming a thermal profile that at least partly overlaps a distribution of the magnetic field at the location of the recording layer and a trailing slope of the magnetic distribution lags behind or coincides with a trailing slope of the thermal profile. The area of the thermal profile for the purpose of determining overlap with the distribution of the magnetic field is the area where the temperature is higher than the average of room temperature and the maximum temperature in the thermal profile. Similarly, the area of the distribution of the magnetic field is the area where the magnetic field is larger than half the value of the maximum intensity of the magnetic field in the distribution. There is overlap in the above sense if both areas overlap.
A hybrid recording apparatus is known from inter alia the article by H. Katayama, S. Sawamura, Y. Ogimoto, J. Nakajima. K. Kojima and K. Ohta, published in Proc. of MORIS""99, J. Magn. Soc. Jpn. 23, Suppl. S1, 233 (1999). A disadvantage of the known apparatus is that the storage life of the storage media recorded with it are shorter than can be derived from the stability of recorded bits at room temperature.
In the third aspect a hybrid recording apparatus is provided that can record storage media having a longer storage life of the recorded data.
This is achieved if the apparatus is characterised in that the radiation distribution is elongate with a longest dimension in the track direction. The solution is based on the insight that the relatively short storage life of the information written with the known apparatus is caused by the use of a spot that is wider than the track width. This means that xe2x80x98thermalxe2x80x99 side erasure occurs, reducing the storage life of data recorded in tracks neighbouring the one currently being recorded. A roughly rectangular (pulsed) radiation spot is proposed, e.g. by using a planar wave guide of which the smallest dimension of the radiation spot is in the track width direction and the longest dimension in the direction of the track. The width of the laser spot is about equal to or smaller than the track pitch, and the medium has optimal thermal performance, so avoiding erasure by thermal decay in neighbouring tracks during writing. An extended magnetic field is generated at the trailing side of the laser (no thermal decay; no straight transition) or a magnetic field with a sharp trailing edge is generated before the trailing edge of the laser spot (small thermal decay; straight transition).
In a preferred embodiment of the apparatus, the recording head includes a planar optical waveguide for forming the radiation field, the waveguide having an elongate exit window facing the location of the recording layer, the longest dimension of the exit window being oriented in the track direction. In a specific embodiment of this recording head, the exit window includes a trailing side and a leading side, the trailing side being closer to the location of an entrance plane of the storage medium than the leading side. Preferably, the ratio of the length of the largest dimension of the exit window over the length of its shortest dimension is larger than two.
The recording head preferably includes a magnetic read head having a width about equal to or smaller than the smallest dimension of the exit window. In a special embodiment the recording head includes a magnetic write head having a width substantially equal to or smaller than the smallest dimension of the exit window.
In an advantageous embodiment the apparatus includes a control unit for controlling the radiation field and the magnetic field, adapted for pulsing the radiation field, preferably synchronously with the magnetic field.
A storage medium suitable for use in the apparatus according to the third aspect of the invention includes a substrate and a magnetic recording layer arranged on the substrate, the substrate having a large heat capacity and small resistivity to serve as a heat sink for the magnetic recording layer, the magnetic recording layer having a small heat capacity and a thermal resistivity lower in the direction of the heat sink than in the plane of the magnetic recording layer.
A fourth aspect of the invention relates to a storage medium for thermally-assisted magnetic recording including a recording layer for writing information in the form of magnetisation transitions by heating the recording layer to a temperature Tw during a time tw, the recording layer having an anisotropy field Hk, a saturation magnetisation Ms, a remanence Mr and a reversal attempt rate f0. The invention also relates to an information storage system including said storage medium and an apparatus for recording information on the medium.
Hybrid recording media are known from inter alia the article by H. Katayama, S. Sawamura, Y. Ogimoto, J. Nakajima. K. Kojima and K. Ohta, published in Proc. of MORIS""99, J. Magn. Soc. Jpn. 23, Suppl. S1, 233 (1999).
In the fourth aspect hybrid recording media are provided having a higher areal density than the known recording media.
This is achieved if in the recording medium the anisotropy field and the saturation magnetisation have a temperature dependence complying with
Hk(Ts)Ms(Ts)=C Hk(Tw)Ms(Tw)
where Ts is a storage temperature of the recording medium smaller than the write temperature Tw and C is a constant larger than or equal to 4. The areal density increase over conventional magnetic recording is than a factor of sqrt (C)=2 or more.
Preferably, the recording medium is characterised by a temperature dependence of the anisotropy field and the saturation magnetisation complying with                     H        k            ⁡              (                  T          s                )              ⁢                  M        s            ⁡              (                  T          s                )              ≥            (                                    T            s                    ⁢                      ln            ⁡                          (                                                t                  s                                ⁢                                  f                  o                                            )                                                            T            w                    ⁢                      ln            ⁡                          (                                                t                  w                                ⁢                                  f                  o                                            )                                          )        ⁢                  H        k            ⁡              (                  T          w                )              ⁢                  M        s            ⁡              (                  T          w                )            
where Ts is the storage temperature of the storage medium and ts is a decay time equal to 3 107s. The parameter tw is the write time, i.e. the time that the written bits are subject to heating to approximately Tw. The relation gives the required medium parameters to achieve a storage lifetime equal to ts by reducing thermally-activated magnetisation reversals in the recording layer. Since the first factor after the inequality sign is proportional to the gain in areal density of hybrid recording over conventional magnetic recording, the inequality also gives the areal density achievable in hybrid recording as a function of write and storage parameters and the stability parameter f0. If the parameters of the medium are chosen using the  greater than  sign in the inequality, the read signal that can be obtained from the medium will be higher than when the = sign is used.
The stability of the recorded magnetisation transitions is improved if Hk(T) Ms(T) is monotonically increasing when the temperature T of the recording layer decreases from Tw to Ts.
For a thermally-assisted recording system with a read temperature Trxe2x89xa7Ts for reading recorded information, the storage medium has parameters Hk(Tr) and Ms(Tr) that comply preferably with                     H        k            ⁡              (                  T          r                )                            H        k            ⁡              (                  T          w                )              ≥                    M        s            ⁡              (                  T          r                )                            M        s            ⁡              (                  T          w                )              ≥            (                                    T            r                    ⁢                      ln            ⁡                          (                                                t                  r                                ⁢                                  f                  o                                            )                                                            T            w                    ⁢                      ln            ⁡                          (                                                t                  w                                ⁢                                  f                  o                                            )                                          )              1      2      
where tr is the time the recording layer stays on temperature Tr during reading. For safety this time may be chosen equal to ts. Note that tr=ts when Tr=Ts. In a specific embodiment, the read temperature Tr is larger than Ts, wherein Msxe2x89xa00 at Ts. The storage medium has preferably monotonically increasing values of Hk(T) and Ms(T) for a temperature T decreasing from Tw to roughly Tr, and Hk(T) monotonically increasing and Ms(T) monotonically decreasing for the temperature T decreasing from roughly Tr to Ts.
In a preferred embodiment the storage medium complies with                     H        k            ⁡              (        T        )                            M        s            ⁡              (        T        )              ≥      1    ⁢          xe2x80x83        ⁢    or    ⁢          xe2x80x83        ⁢                            H          c                ⁡                  (          T          )                                      M          r                ⁡                  (          T          )                      ≥  0.5
at every temperature between Tw and Ts and Mr is the remanence of the recording layer.
The storage medium advantageously complies with   1.4  ≤                    H        k            ⁡              (        T        )                            M        s            ⁡              (        T        )              ≤      2    ⁢          xe2x80x83        ⁢    or    ⁢          xe2x80x83        ⁢    0.7    ≤                    H        C            ⁡              (        T        )                            M        r            ⁡              (        T        )              ≤  1
at every temperature between Tw and Ts.
The storage medium suitable for reading at a temperature Tr substantially equal to Ts, is preferably characterised in that Hk(T) and Ms(T) are monotonically increasing for a temperature T decreasing from Tw to Ts.
Preferably, Hc(Tw) greater than 240 kA/m and/or Hc(Ts) greater than 480 kA/m.
The storage temperature Ts is preferably substantially equal to 300 K.
The write temperature Tw complies preferably with Ts+100 K less than Tw less than 800 K. More preferably, Ts+100 K less than Tw less than 570 K.
The storage medium has preferably a value of the write time tw that complies with 10 ns less than tw less than 10 xcexcs.
The reversal attempt rate f0 of the recording layer is preferably substantially equal to 109/s.
An information storage system including a storage medium having a recording layer with an anisotropy field Hk, a saturation magnetisation Ms and a reversal attempt rate f0, and an apparatus for thermally-assisted magnetic recording of information in the form of magnetisation transitions in the recording layer by heating the recording layer, the apparatus including a recording head for forming a radiation field and a magnetic field at the position of the recording layer to be recorded and a control unit for controlling the radiation field and the magnetic field, in which the control unit is adapted for controlling the radiation beam such that the recording layer is heated to a temperature Tw during a time tw, and that a temperature dependence of the anisotropy field Hk and the saturation magnetisation Ms complies with                     H        k            ⁡              (                  T          s                )              ⁢                  M        s            ⁡              (                  T          s                )              ≥            (                                    T            s                    ⁢                      ln            ⁡                          (                                                t                  s                                ⁢                                  f                  o                                            )                                                            T            w                    ⁢                      ln            ⁡                          (                                                t                  w                                ⁢                                  f                  o                                            )                                          )        ⁢                  H        k            ⁡              (                  T          w                )              ⁢                  M        s            ⁡              (                  T          w                )            
where Ts is the storage temperature of the storage medium and ts is a decay time equal to 3 107s.